1. Field of the Invention
The present invention relates to a near-field light generator including a plasmon generator and a waveguide, and to a thermally-assisted magnetic recording head including the near-field light generator.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head section including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head section including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the magnetic recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the aforementioned problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a magnetic recording medium having high coercivity. When writing data, a write magnetic field and heat are applied almost simultaneously to the area of the magnetic recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light. The light for use in generating near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near the medium facing surface of the slider. The medium facing surface is the surface to face the magnetic recording medium. The waveguide includes a core through which light propagates, and a cladding provided around the core. The cladding has a refractive index lower than that of the core.
In a thermally-assisted magnetic recording head including a plasmon generator and a waveguide, the plasmon generator and the core of the waveguide are disposed close to each other in the vicinity of the medium facing surface. Thermally-assisted magnetic recording heads having such a configuration are disclosed in, for example, U.S. Patent Application Publication Nos. 2011/0164334 A1, 2011/0170381 A1 and 2011/0235480 A1, and U.S. Pat. No. 8,170,389 B1.
Part of the energy of the light guided to the plasmon generator through the waveguide is transformed into heat in the plasmon generator. The plasmon generator thus rises in temperature during the operation of the thermally-assisted magnetic recording head. A considerable rise in temperature of the plasmon generator may cause the following phenomenon, possibly impairing the reliability of the thermally-assisted magnetic recording head. Specifically, there is a possibility that the plasmon generator may expand and protrude from the medium facing surface to cause damage to the magnetic recording medium or to the plasmon generator itself. Furthermore, a considerable rise in temperature of the plasmon generator may also cause the plasmon generator to be deformed due to migration of atoms of the material forming the plasmon generator. This may cause the plasmon generator to become unable to provide a desired heating capability. Thus, it is critical with thermally-assisted magnetic recording heads to prevent a rise in temperature of the plasmon generator.
To prevent a rise in temperature of the plasmon generator, the plasmon generator is preferably formed of a material that has a low dielectric loss. Forming the plasmon generator of a material having a low dielectric loss will reduce the amount of absorbed energy caused by dielectric loss inside the plasmon generator, and will thereby reduce the amount of heat generated by the plasmon generator. The relative permittivity of a material is expressed as n2−k2+i2nk, where n and k are the refractive index and the extinction coefficient of the material, respectively. The greater the imaginary part 2nk of the relative permittivity, the greater the dielectric loss becomes. Thus, the plasmon generator is preferably formed of a material that has a reduced value of 2nk. Examples of such a material include one metal selected from the group consisting of Au, Ag, Cu and Al, and an alloy containing at least one of these metals.
However, Au, Ag, Cu, and Al are relatively soft metals. A plasmon generator formed of any of these materials thus has a drawback that, when the plasmon generator is at an elevated temperature, growth or aggregation of crystal grains will occur due to internal strain, relaxation of surface energy, or stress migration caused by thermal stress, and this will cause the plasmon generator to be susceptible to deformation. Such deformation of the plasmon generator tends to occur particularly when the plasmon generator has low adhesion to a material in contact therewith.
A thermally-assisted magnetic recording head including a plasmon generator and a waveguide is often configured so that the material forming the cladding in contact with the core is brought into contact with part of the outer surface of the plasmon generator other than the end face located in the medium facing surface. The cladding is often formed of Al2O3 (hereafter, also referred to as alumina) or SiO2. Au and Ag have low adhesion to alumina and SiO2. Thus, if a plasmon generator formed of Au or Ag is in contact with a cladding formed of alumina or SiO2, deformation of the plasmon generator tends to occur as mentioned above.
U.S. Patent Application Publication No. 2011/0164334 A1 discloses MgO and MgF2, in addition to alumina and SiO2, as materials usable for the cladding. However, since MgO is higher in refractive index than alumina and SiO2, using MgO as the material of the cladding to directly contact the core will make the wave guide efficiency of the waveguide lower than in the case of using alumina or SiO2 as the material of the cladding.
On the other hand, MgF2 is a material that has a very low thermal conductivity. Thus, if MgF2 is in contact with the plasmon generator, dissipation of heat generated in the plasmon generator will be inhibited to cause a rise in temperature of the plasmon generator.
It has thus been conventionally difficult to prevent the plasmon generator from being deformed due to a rise in temperature thereof, without degrading in the wave guide efficiency of the waveguide.